Under some circumstances, RF energy application may be accompanied by an electric discharge. For example, if a metallic spoon is put in a microwave oven, operation of the oven may generate a spark between the spoon and the walls of the oven. This spark is a kind of electric discharge. An electric discharge may be caused by ionization of some of the medium in which the discharge happens. The discharge may be expressed by electrical currents going through air or other insulator between two conductors. The discharge may have different properties depending on the intensity of the electric field that produced it. At fields of lower intensity, a dark discharge may be generated. At somewhat higher fields, a corona discharge may take place. The corona discharge may be accompanied with emission of visible photons. At higher fields, sparks or arcs may form, in a process known as arcing.